Numerous configurations have been attempted in the history of aviation to enable aircraft to take off and land vertically.
The helicopter configuration enables stationary flight to be performed under satisfactory control, but that is to the detriment of cruising speed. Research has thus been undertaken to improve hovering control of airplanes. Nevertheless, the solutions that have been found so far have been of moderate efficiency and have required complex modifications either to propulsion systems or to engines.
Document U.S. Pat. No. 5,054,716 discloses a convertible aircraft where each of the rotors together with its control and engine means constitutes a tilting assembly referred to as a “tilt rotor”. In that configuration, each engine must be capable of operating both in a substantially horizontal position and also in a vertical position, and that raises numerous problems associated either with the engine or with the cabling or the hydraulic and fuel ducts that need to pass through a tilting pivot. Furthermore, since the engines are generally turboengines, the gas jets from their nozzles have the drawback in helicopter mode of damaging the surface of take-off and landing areas.
Document U.S. Pat. No. 3,360,217 discloses a convertible aircraft having four tilt fans. In that configuration, the engines are disposed on the fuselage and remain in the same orientation regardless of the stage of flight. Passing from forward flight referred to as “airplane” mode to a helicopter mode is done by tilting the tilt fans only. Consequently, that makes it possible to avoid all problems associated with tilting an engine while it is running.
However, using four tilt fans is particularly penalizing insofar as the aircraft must have sufficient engine power to be capable of driving all four tilt fans. In addition, it goes without saying that the weight of the device is considerable, thereby correspondingly reducing the payload of the aircraft.
From those considerations, firstly it can be seen that the tilt rotor configuration is particularly difficult to implement and make suitable for industrial manufacture, because of the difficulties associated with tilting an engine. Secondly, it can be seen that the tilt fan configuration is easier to implement and does not present the above-described drawbacks, but that using four tilt fans increases the weight of the airplane in penalizing manner.